In U.S. Pat. No. 3,866,052, commonly-assigned herewith, methods for use in defining the spatial location of a point on an object surface involve the practice of projecting radiant energy onto the surface through digital mask structure, i.e., masks having respectively discrete energy transmissive and energy non-transmissive extents. In the preferred practice of the U.S. Pat. No. 3,866,052, differently configured masks establish diverse irradiated segments of the projection field and are used successively. Records made in corresponding succession are examined to identify those records in the succession of records which contain, or do not contain, a representation of the illumination of each surface point of interest. Digital signal patterns are generated from the record succession unique for surface points which are in different projection field segments. These signals, together with information as to the positional location of surface points in the two-dimensional records and the spatial location of the recording lens node enable transition from two-dimensional data to spatial position determination. The lens node and record positional information establish a line of sight to the surface point and the digital signals identify the location of such point along the line of sight.
The degree of resolution in spatial point location in the U.S. Pat. No. 3,866,052 digital practice is dependent on the number of irradiated segments established by the most expanded digital mask used. The measure of error is accordingly related to the narrowest width segment and surface points closely disposed within such segment are not distinguishable from one another. In practice under the U.S. Pat. No. 3,866,052, operative mask segment subdivision is thus selected in accordance with desired resolution. The succession of projections and records is expanded to attain high resolution with accompanying expenditure of practice time and record making time and expense.
In an alternative practice set forth in the U.S. Pat. No. 3,866,052, energy projection is made through a single mask having plural unique frequency-transmissive extents. Disparity between projection field segments in a common record is achieved since each segment has unique frequency character. On the other hand, differentiation between surface points closely located in a common segment is not provided, and ultimate resolution is again determined by the number of mask segment subdivisions.